This invention relates generally to the field of fuel nozzles and, more particularly, to a combustor and associated fuel nozzle having improved fuel concentration profile characteristics.
Combustion engines are machines that convert chemical energy stored in fuel into mechanical energy useful for generating electricity, producing thrust, or otherwise doing work. These engines typically include several cooperative sections that contribute in some way to this energy conversion process. In gas turbine engines, air discharged from a compressor section and fuel introduced from a fuel supply are mixed together and burned in a combustion section. The products of combustion are harnessed and directed through a turbine section, where they expand and turn a central rotor.
A variety of combustor designs exist, with different designs being selected for suitability with a given engine and to achieve desired performance characteristics. One popular combustor design includes a centralized pilot nozzle and several main fuel injector nozzles arranged circumferentially around the pilot nozzle. With this design, the nozzles are arranged to form a pilot flame zone and a mixing region. During operation, the pilot nozzle selectively produces a stable flame which is anchored in the pilot flame zone, while the main nozzles produce a mixed stream of fuel and air in the above-referenced mixing region. The stream of mixed fuel and air flows out of the mixing region, past the pilot flame zone, and into a main combustion zone, where additional combustion occurs. Energy released during combustion is captured by the downstream components to produce electricity or otherwise do work.
In one version of this type of combustor, two types of combustion occur: high-swirl-combustion occurs in the pilot flame zone, with low-swirl-number combustion occurring in the main combustion zone. As is known in this field, high-swirl-number combustion is characterized by relatively-compact flames, with high rates of rotation and relatively-low rates of longitudinal propagation. Low-swirl-number combustion, conversely, is characterized by flames which are relatively more spread out. By combining high swirl number combustion in the pilot flame zone with low swirl number combustion elsewhere, this type of combustor provides stable and predictable operation and a high degree of monitorability. As a result, this type of combustor is suitable for use across a wide range of operating conditions. Additionally, by providing a combustion scheme which yields a wide-spread distribution of energy within the combustion chamber, this type of combustor is also resistant to thermo-acoustic excitations. These combustors also present a relatively-long pre-combustion mixing path for the fuel and air which helps ensure even-temperature burning and reduced emissions levels. Accordingly, this type of combustor is a popular choice for use in industrial turbine engines.
In order to ensure optimum performance of this type of combustor, it is generally preferable that the internal fuel-and-air streams are well-mixed, to avoid localized, fuel-rich regions. Combustion of over-rich pockets of fuel and air leads to high-temperature combustion that produces high levels of unwanted NOx emissions. As a result, efforts have been made to produce combustors with essentially-uniform distributions of fuel and air. Swirler elements, for example, are often used to produce a stream of fuel and air in which air and injected fuel are evenly mixed.
Unfortunately, while attempts to reduce emissions by uniformly distributing fuel and air are effective in some cases, they are not suitable with all combustors. For example, combustors like the ones described above, which combine high-swirl-number combustion in a pilot zone with low-swirl-number combustion in a main combustion zone, can actually suffer increases in unwanted emissions and acoustic resonance problems when used with nozzles that produce uniform distributions of fuel and air. In this type of combustor uniformly distributed mixtures of fuel and air lead to flame holding at the main nozzle tips which, in addition to increasing unwanted emissions and acoustic problems, also introduces the need for nozzle tip cooling and increases the risk of dangerous flashback. Therefore, while efforts to improve performance through uniformly distributing fuel and air are effective in some settings, they can actually reduce the performance of some combustors.
Accordingly, there remains a need for a performance-enhancing nozzles suitable for use in combustors which combine high-swirl-number combustion in a pilot zone with low-swirl-number combustion in a main combustion zone. The nozzle should eliminate combustion outside the mixing zone immediately downstream of the nozzle, without negatively impacting the overall performance of the combustor. The nozzle should produce a radially-biased fuel concentration profile which reduces the tendency for flame holding at the nozzle tip. The nozzle should also provide the desired fuel concentration profile over a wide range of operating conditions, without regard to fluctuating fuel and air inputs.
The instant invention is a performance-enhancing nozzle suitable for use in combustors which combine high-swirl-number combustion in a pilot zone with low-swirl-number combustion in a main combustion zone. The nozzle includes a fuel delivery member adapted for fluid communication with a source of fuel and a flow conditioning member that includes at least one fuel exit port which is in fluid communication with the fuel supply and adapted to ensure that the region adjacent the nozzle tip remains flame free. In one aspect of the invention, the nozzle produces a fuel concentration profile characterized by a radially-outward region that is flammable and a radially-inward region that is substantially non-flammable. In another aspect of the invention, the flow conditioning element includes a radially-inboard first portion and a radially outward second portion, with the fuel exit ports being disposed in the second portion. In another aspect of the invention, the flow conditioning element is characterized by a swirl number lower than about 0.6. In another aspect of the invention, the exit ports may be characterized as high-momentum, having a design ratio pressure of greater than about 1.1. In another aspect of the invention, the nozzle is part of a combustor which produces high-swirl-number combustion in a pilot zone and low-swirl-number combustion in a main combustion zone.
Accordingly, it is an object of the present invention to provide a fuel nozzle that eliminates combustion outside a mixing zone immediately downstream of the nozzle, without negatively impacting the overall performance of the combustor.
It is another object of the present invention to provide a nozzle that produces a radially-biased fuel concentration profile which reduces the tendency for flame holding at the nozzle tip.
It is yet a further object of the present invention to provide a nozzle that produces the desired fuel concentration profile over a wide range of operating modes, without regard to fluctuating nozzle inlet conditions.
It is also an object of the present invention to provide a nozzle that is compatible with previously-installed combustors, allowing the nozzle to be used in retrofit operations.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.